1. Field of the Invention This invention relates to a kind of suppression system of background noise of speech signals and the method thereof. The suppression system of background noise of the invention focuses on the short time and long time characteristics of speech signals and the method thereof.
2. Description of the Prior Art
Voice sound signals are a major data type transmitted in telecommunication systems. During the process of communication, in addition to the voice sounds, background noise of the telecommunication environment also enters into the telephone, and will cause some degree of interference and influences the quality of the telecommunication. In particular, recent rapidly-growing mobile phone use is easily influenced by the background noise. So the technology of suppression of background noise is one important topic relating to quality of current telecommunication systems. There are three kinds of technology commonly used for the suppression of background noise, as follows:
The first method is the method of deleting the noise in the frequency domain. The basic principle of this method is to estimate the energy of the noise within the frequency domain in a segment of non-speech sounds, and then to eliminate the estimated energy of the noise at each frequency in the frequency domain in the speech segments that follow. Although this method is simple, its effect on suppression of background noise is limited, since the statistical characteristics of the general background noise varies with time. This method of suppression of noise is disclosed in U.S. Pat. No. 6,175,602 and 5,742,927.
The second method is the method of deleting the background noise in the time domain. The basic principle of this method is utilization of two microphones to receive the outside signals. The Primary microphone is used to receive the speaker's voice along with the background noise. The secondary microphone is used to receive only the background noise. Thus, the background noise could be estimated through the secondary microphone. Next, by subtracting the estimated background noise from the signal of the first microphone in the time domain, better quality speech signals can be obtained. However, this method requires two microphones and there must be a sufficient distance between these two microphones, which is nearly impossible for mobile phone applications.
The third method is the periodic tracking method. The basic principle of this method is to estimate and track the periods of voice signals first, and next to find the average of the related signals within a few periods. The enhancement of speech is achieved by averaging the delayed and weighted versions of input speech signals, where the delay lengths correspond to the detected pitch periods. Since background noise does not possess the same pitch periods as the original speech, it is cancelled out by this operation. The concept of using subtraction with periodic tracking is disclosed in U.S. Pat. 5,598,158.
It could be found from the above mentioned methods that there still are many drawbacks in the above-mentioned technologies and there is a urgent need for improvement.